This invention relates to a method and an apparatus for generating a multidimensional map of blood velocities in the circulatory system of living species, employing the Doppler shift of backscattered ultrasound from blood. The map is presented in real time on a display device, and the invention relates in particular to the possibility of selecting one or more sample volumes in the display from which the Doppler signal can be presented audibly through a loudspeaker or alike, and the frequency analysis of the Doppler signal can be presented as a function of time together with the moving flow map in real time, representing the time variation of the blood velocities in the sample volumes selected. The display of the blood velocities can also be presented together with a for practical purposes simultaneous image of tissue structures in the system, e.g. the heart muscle.
The field of the invention is ultrasound diagnosis of the cardiovascular system as being done in hospitals and the private physicians office.
There has both been presented scientific papers describing blood flow imaging, and commercial instruments that sweep the ultrasonic beam over a two dimensional field and calculate the Doppler shift as a function of depth to present a two dimensional color coded display of a blood velocity distribution. This invention differs from prior art in the ability to select one or more sample volumes in the displayed map from which one can get a continuous audible presentation of the Doppler signal and a display of the of the spectrum of the Doppler signal representing the blood velocities in the selected sample volumes versus time. The invention also differs from prior art in the method of high-pass filtering to remove backscattered signal from tissue structures, the method of calculating the Doppler shift, and the use of a continuously swept beam from a mechnically steerable transducer to generate the flow map.
The flow map is obtained by sweeping the ultrasonic beam to generate a time sequential measurement in different beam directions. The measurement in each sample volume is therefore intermittent, and to be able to give a continuous presentation of the Doppler signal from selected sample volumes, the invention uses a signal synthesizer which on the basis of short segments of the Doppler signal from a particular sample volume, generates a continuous substitute signal with spectral properties close to the original signal that would have been obtained from continuous measurements on that sample volume, and which can be presented in audible form and analysed instead of the original signal. By this an apparent continuous presentation of the signal from the sample volume can be presented although the measurement from this volume is done intermittently during short intervals with some distance in time.
In the interrupt intervals when measurements are not done in the particular sample volume, one can do Doppler measurements at other directions or one can also generate a pulse echo amplitude image of tissue structures using methods known per se, as can be seen in Norwegian patent application no. 82.1245 and U.S. Pat. No. 4,559,952. This type of time sharing between Doppler blood velocity measurements and pulse echo amplitude imaging allows for optimizing the shape of the transmitted pulse for the two types of measurements.
The interrupt intervals of the Doppler measurements for a particular sample volume are much larger than the correlation time of the Doppler signal. It is then difficult to estimate the true waveform of the missing Doppler signal. Instead one generates a substitute signal with approximately the same correlation properties (spectral properties) as the original signal, i.e. we are not concerned with the phase of the Fourier transform, only the amplitude.
Also we use the signal after the strong components from tissue structures have been removed by filtering, as a basis for the synthesis. This requires less relative accuracy in the estimation since the signal components from tissue structures are much stronger than the components from the blood. If we try to estimate the signal before the filtering, even a small relative error can give signal components comparable in magnitude to the components from blood and thus be very disturbing.
One should note that the Doppler measurements from different beam directions and the generation of the pulse echo amplitude image are done consecutively in time and are thus not truly simultaneous. However, the time interval to do all the measurements over the whole image field (both blood velocity and tissue image) can be made sufficiently short for the image to appear instantaneous on the screen with a frame rate of 10-20 images per sec. Thus for practical purposes a real time display can be obtained.
Although the invention relates to a multidimensional map of the blood flow, one will in most cases use a two dimensional map. The combined two-dimensional Doppler-amplitude image is presented on a suitable screen, for example a color video screen in which the echo amplitude image is for example coded in gray scale, whereas the Doppler image is produced as a color overlay as known per se. A proposal for a color code may be red for velocities towards the transducer and blue for velocities away from the transducer, with dark red and dark blue indicating low velocities, with transitions to light red/yellow and light blue/white for high velocities.
It is also important that the Doppler signal from the selected sample volumes may be presented in audible form, for example in headphones or a loudspeaker. The operator carrying out the investigation will to a large extent take advantage of the information contained in the audible signal. Therefore, it has much significance that the audibly presented Doppler signal is not substantially disturbed or distorted from what it is for a noninterrupted Doppler measurement.
The object of the invention is to provide a method and an apparatus which may quickly detect abnormalities of the blood flow in the heart and vessels. The investigation may be carried out by applying an ultrasonic transducer at the skin surface, thereby making the method noninvasive, in contrast to X-ray and radio isotope methods in which it is necessary to make injections in the body. The method may also be used in an invasive manner during operations, and the injection of an ultrasonic contrast liquid may also be employed in order to obtain a better Doppler image. In order to determine the absolute velocity of the blood in Doppler methods, there must be made corrections for the angle between the velocity vector of the scattering elements and the sound beam. By imaging the blood flow in a plane, the angle in this plane may be determined, and corrections of the velocity values for the angle in this plane can be made. By rotating the plane until a maximum velocity is obtained, the complete velocity vector will lie in the plane, and thereby the absolute velocity may be completely determined.
The present invention is based upon the use of a pulse type ultrasonic beam. In order to produce the above two-dimensional Doppler image the ultrasonic beam (1-20 MHz) is swept in several directions in a plane. For measurements on the heart this advantageously takes place with a sector sweeping of the beam, whereas a linear sweep or a combination of linear and sector sweeping may be preferable when measurements on pheripheral vessels, abdominal organs and fetuses are made.
In the claims there are set forth closer statements of the method and the apparatus according to the invention.